-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 On Sun, 9 Oct 2011 15:46:20 -0400 V G wrote: > My FPGA is the Xilinx Spartan 3E 500K gates. >=20 > 1. What is the difference between block RAM and distributed RAM? I > know registers can be implemented as flip flops, but are the > implemented in logic or in the RAM? What's going on here exactly? Block RAM is a large chunk of RAM built in a single place on the chip. Distributed RAM is RAM implemented by reusing the LUT (*NOT* the flip-flop!) in a logic block as a small number of RAM bits (hence it's "distributed" throughout the chip, in the SLICEMs). Neither of these are the same as the logic block's flip-flop, which only stores one bit but provides that storage in addition to using the LUT for computation. As an aside, in addition to being used as RAM or a computational LUT, the LUTs in SLICEMs can also be used as shift registers. This is all set up at bitstream load time; the purpose of a particular LUT can't be changed during execution. Also, LUTs that are in SLICELs, rather than SLICEMs, can't be used as distributed RAM nor as shift registers. The gory details are all in chapters four through seven of the Spartan 3 user guide. Also, you may get more answers on the USENET group comp.arch.fpga than here, this being the PICLIST and all. There are some real experts there, including people who work at Xilinx and Altera. > 2. When I look at the Spartan 3 data sheet, it sizes the different > parts in number of gates and gives an "equivalent number of logic > cells". When I look at the Spartan 6 data sheet, it sizes the parts > in logic cells and gives "equivalent number of gates". What's going > on here? Are both the series structurally similar? Don't both have > logic cells and slices and so on? The Spartan 6 series' logic blocks are similar in structure, but the LUTs have six address inputs instead of four and therefore are "worth more" in terms of equivalent gates. Chris -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.17 (GNU/Linux) iEYEAREDAAYFAk6SMU8ACgkQXUF6hOTGP7c6JQCcDV+Bxze7Tmgcc+so/qNzsipO u1IAn0W7buI+Lp/nRPAeynF7ohhodcga =3Ddgre -----END PGP SIGNATURE----- --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .